High-speed, silicon-based electro-optic modulator with feedback control

ABSTRACT

An electro-optic modulator arrangement for achieving switching speeds greater than 1 Gb/s utilizes pre-emphasis pulses to accelerate the change in refractive index of the optical waveguide used to form the electro-optic modulator. In one embodiment, a feedback loop may be added to use a portion of the modulated optical output signal to adjust the magnitude and duration of the pre-emphasis pulses, as well as the various reference levels used for modulated. For free carrier-based electro-optic modulators, including silicon-based electro-optic modulators, the pre-emphasis pulses are used to accelerate the movement of free carriers at the transitions between input signal data values.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No.60/468,885, filed May 8, 2003.

TECHNICAL FIELD

The present invention relates to silicon-based electro-optic modulatorsand, more particularly, to the provision of high speed (e.g., greaterthan 1 Gb/s) modulators by incorporating channel equalization techniquesinto the modulator and associated electronic drive circuit.

BACKGROUND OF THE INVENTION

Optical transmission systems are generally based on one of two methodsof modulation of an optical signal, either direct modulation or externalmodulation. In the first of these methods, the bias current applied to alaser is modulated, turning the laser “on” and “off”. The disadvantageof this method is that when higher switching speeds are required, thedynamic behavior of the semiconductor material of the laser itselfintroduces distortion, primarily in the form of chirp. Externalmodulation of an optical signal with a modulating electrical signalproduces a modulated optical output signal with significantly reducedchirp, and external modulators have become preferred for high speedapplications. In particular, electro-optic modulators such as MachZehnder interferometers are typically used for high speed applications.

For many years, external modulators have been made out of electro-opticmaterial, such as lithium niobate. Optical waveguides are formed withinthe electro-optic material, with metal contact regions disposed on thesurface of each waveguide arm. The application of a voltage to a metalcontact will modify the refractive index of the waveguide regionunderneath the contact, thus changing the speed of propagation along thewaveguide. By applying the voltage(s) that produce a π phase shiftbetween the two arms, a nonlinear (digital) Mach-Zehnder modulator isformed. In particular, the optical signal is launched into thewaveguide, and the 1/0 electrical digital signal input is applied to thecontacts (using proper voltage levels, as mentioned above). The opticaloutput is then “modulated” to create an optical 1/0 output signal. Asimilar result is possible with a linear (analog) optical output signal.

Although this type of external modulator has proven extremely useful,there is an increasing desire to form various optical components,subsystems and systems on silicon-based platforms. It is furtherdesirable to integrate the various electronic components associated withsuch systems (for example, the input electrical data drive circuit foran electro-optic modulator) with the optical components on the samesilicon substrate. Clearly, the use of lithium niobate-based opticaldevices in a such situation is not an option. Various other conventionalelectro-optic devices are similarly of a material (such as III-Vcompounds) that are not directly compatible with a silicon platform.

A significant advance in the ability to provide optical modulation in asilicon-based platform has been made, however, as disclosed in ourco-pending application Ser. No. 10/795,748, filed Mar. 8, 2004. FIG. 1illustrates one exemplary arrangement of a silicon-based modulatordevice as disclosed in our co-pending application. In this case, a“MOSCAP” structure 1 in terms of a doped (i.e., “metal-like”) siliconlayer 2 (usually polysilicon) is disposed over a doped portion of arelatively thin (sub-micron) surface layer 3 of a silicon-on-insulator(SOI) wafer 4, this thin surface layer 3 often being referred to in theart as the “SOI layer”. A thin dielectric layer 5 is located between thedoped, “metal”-like” polysilicon layer 2 and the doped SOI layer 3, withthe layers disposed so that an overlap is formed, as shown in FIG. 1, todefine an active region of the device. Free carriers will accumulate anddeplete on either side of dielectric layer 5 as a function of voltagesapplied to SOI layer 3 (VREF3) and/or polysilicon layer 2 (VREF2). Themodulation of the free carrier concentration results in changing theeffective refractive index in the active region, thus introducing phasemodulation of an optical signal propagating along a waveguide formedalong the active region (the waveguide being in the directionperpendicular to the paper).

As of now, such silicon-based electro-optic modulators have beenoptimized to minimize the optical loss. The optical loss is controlledby reducing optical signal absorption along the extent of the waveguide.Since the absorption is directly related to the carrier doping density,a minimal optical loss requires a minimal dopant density in bothpolysilicon layer 2 and SOI layer 3. However, this optical lossspecification runs in direct opposition to the desire for high speedoperation. That is, to provide a high speed (i.e., switching speedgreater than 1 Gb/s) device, a relatively high doping density isrequired. Inasmuch as system requirements are even now moving toward 10Gb/s, there is a strong need to increase the switching speed of asilicon-based electro-optic modulator, without sacrificing optical powerto attain high speed operation.

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the presentinvention, which relates to silicon-based electro-optic modulators and,more particularly, to the provision of high speed (e.g., greater than 1Gb/s) modulators by incorporating channel equalization techniques intothe modulator and associated electronic drive circuit.

In accordance with the present invention, channel equalization isachieved by developing a doping profile for the device terminals thatprovides the desired limit of optical loss. For a given doping profile,there will be an associated maximum switching speed at which themodulator will function when using a simple drive circuit (i.e., atransition between two reference voltage levels, such as VDD and VSS, toswitch between an optical “0” and an optical “1”). To increase theswitching speed in accordance with the present invention, pre-emphasisvoltages are applied during transitions between “1” and “0” (fallingedge transitions), as well as between “0” and “1” (rising edgetransitions), where the pre-emphasis voltage will accelerate chargingand discharging of the MOSCAP (or modulator active capacitance), thusreducing the fall and rise times respectively between states. It shouldbe understood that the output impedance of such pre-emphasis circuitsshould be as low as possible and in fact approach a voltage source forthose modulators driven by voltage type signals. Therefore, thepre-emphasis circuits and techniques described hereinbelow also allowfor an impedance transformation from the data source (usually a 50 Ωimpedance) to a much lower value (certainly less than 25 Ω and usuallyabout 1 Ω), thus approaching an ideal voltage source.

In one embodiment of the present invention, the optimum pre-emphasisvoltage levels and pulse durations may be defined during manufacture, ona device-by-device basis, and stored in a microprocessor-related memory(or other memory type device) co-located with the modulator. In afurther advance of this embodiment, a feedback technique may use a tableof reference voltages and associated pre-emphasis voltage/durationvalues (stored in the look-up table), where as modulation conditionschange (i.e., temperature, supply voltage variations, lifetime agingetc.), the optimum parameter values may be selected from the database toadjust the device performance. Finally, the use of pre-emphasis toextend the channel bandwidth reduces the pattern dependent jitter byincreasing the bandwidth of the channel. In particular, a portion of theoptical output signal may be tapped off and analyzed to determine thenecessary changes.

A preferred layout arrangement of the present invention utilizes aplurality of separate contact points along the length of the polysiliconlayer and the SOI layer in the contact region. Since the speed of lightis finite in silicon, the “flight time” of the optical signal along thelength of the modulator may become a significant portion of the bitperiod. Thus, by fanning out (i.e., distributing) the electrical signalinput along the extent of the active region, the entire waveguide isessentially energized simultaneously such that all parts of thewaveguide instantaneously see the change in voltage.

Other and further advantages, embodiments and features of the presentinvention will become apparent during the course of the followingdiscussion and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 is an exemplary arrangement of a silicon-based modulator device;

FIG. 2(a) illustrates, in simplified block diagram form, abandwidth-limited nonlinear channel, with FIG. 2(b) illustrated the samechannel, but showing the use of an equalizer/pre-emphasis circuit of thepresent invention to improve the optical output characteristics of ahigh speed optical modulator;

FIG. 3 illustrates, in simplified block diagram form, a linear channelequalization arrangement, the contrast between a distortion limitedchannel arrangement (FIG. 3(a)) and an arrangement utilizing anequalizer/pre-emphasis circuit in accordance with the present invention(FIG. 3(b)) to linearize the modulator channel;

FIG. 4 illustrates a simplified block diagram for a Mach Zehnderinterferometer modulator showing the individual accumulation anddepletion arms of the design;

FIG. 5 contains a set of reference voltages and drive patternsassociated with the bandwidth-limited channel arrangement of FIG. 2(a)and the distortion-limited channel arrangement of FIG. 3(a);

FIG. 6 contains an exemplary set of reference voltages and drivepatterns including pre-emphasis that can be used to accomplish increasedswitching speed (greater than 1 Gb/s) in accordance with the presentinvention;

FIG. 7 is a top view, in simplified form, of an exemplary Mach-Zehnderelectro-optic modulator formed in accordance with the present inventionto provide for high speed (greater than 1 Gb/s) operation;

FIG. 8 is a preferred embodiment of the modulator of FIG. 7, in terms ofthe voltage levels used to define the reference voltages and thepre-emphasis voltages;

FIG. 9 is a diagram of the reference voltages and drive patternsassociated with the arrangement of FIG. 8;

FIG. 10 illustrates, in block diagram form, an exemplary modulatorarrangement of the present invention, including a feedback loop betweenthe modulator output and the equalizer/pre-emphasis circuit;

FIG. 11 illustrates another embodiment of a feedback arrangement thatmay be used with an electro-optic modulator formed in accordance withthe present invention, with a tapped-off output signal converted by aphotodiode into an analog electrical signal;

FIG. 12 illustrates an exemplary absorption-based modulator that may usethe pre-emphasis technique of the present invention to increase theswitching speed of the modulator;

FIG. 13 is a graph of the reference voltages and drive patternsassociated with the arrangement of FIG. 12;

FIG. 14 contains a plot illustrating the improvement in switching speedof an electro-optic modulator using equalization/pre-emphasis inaccordance with the present invention associated with the accumulationcase;

FIG. 15 contains a plot illustrating the improvement in switching speedof an electro-optic modulator using equalization/pre-emphasis inaccordance with the present invention associated with the depletioncase; and

FIG. 16 illustrates the change in carrier concentration performance as afunction of time with and without applying pre-emphasis pulses.

DETAILED DESCRIPTION

FIG. 2 illustrates, in simplified form, a nonlinear channel equalizationarrangement showing the use of an equalizer/pre-emphasis circuit of thepresent invention to improve the optical output characteristics of ahigh speed optical modulator. FIG. 2(a) illustrates a bandwidth limitedchannel arrangement, where an electrical input data signal from a datasource 10 is applied as the electrical input to a silicon-basedmodulator structure 12, such as the modulator illustrated in FIG. 1. Itis to be understood that the theory of the present invention is equallyapplicable to any type of electrically driven optical modulator, thatis, a modulator where the refractive index properties of an opticalwaveguide are changed by modulating an electrical input signal toproduce a modified optical output signal. Such electro-optic modulatorsinclude, but are not limited to, free carrier-based modulators,particularly silicon-based electro-optic modulates.

Referring back to FIG. 2, an optical input from a light source 14 isapplied as the second input to modulator 12, where as discussed in ourabove-cited co-pending application, the optical signal is coupled intothe relatively thin SOI layer (such as SOI layer 3 of FIG. 1) andthereafter propagates along the active region of the waveguidestructure. Once the switching speed of the electrical input signal fromsource 10 goes above a certain value (for example, above 1 Gb/s), thefree carriers in the silicon-based modulator cannot charge or dischargethe modulator fast enough to completely change state during the bitperiod. At this point, therefore, the modulator performance begins todegrade, and the optical output signal becomes distorted, as shown byelement 16 in FIG. 2(a).

FIG. 2(b) illustrates, in simplified form, the solution as proposed bythe present invention, where an equalizer/pre-emphasis circuit 18 isdisposed between the electrical input source 10 and modulator structure12. Equalizer/pre-emphasis circuit 18 is configured to recognize eachtransition between the logic levels in the input signal, and insert anadditional “boost” to the voltage applied at the transition. Thispre-emphasis voltage pulse functions to accelerate the free carriermovement, so that essentially all of charging or discharging hascompleted by the end of the bit period. Therefore, the optical outputsignal, as shown by element 20, remains clean, with sharp,clearly-defined transitions between optical “1” and optical “0”.Equalizer/pre-emphasis circuit 18 thus allows for the bandwidth ofmodulator structure 12 to be extended without increasing the opticalloss of the system. Indeed, the doping profile within the active regioncan remain at a relatively low level (for example, 1×10¹⁹ cm⁻³) andstill provide optical switching greater than 1 Gb/s.

As mentioned above, the channel equalization technique of the presentinvention is equally applicable to linear systems, utilizing an analoginput signal (for example, an amplitude modulated signal). FIG. 3illustrates a simplified linear channel equalization arrangement inblock diagram form, the contrast between a distortion limited channelarrangement (FIG. 3(a)) and an arrangement utilizing anequalizer/pre-emphasis circuit in accordance with the present invention(FIG. 3(b)) to linearize the modulator channel. In this case, modulator12 and optical source 14 are the same as those associated with FIG. 2. Alinear electrical signal source 22 is illustrated as being applied asthe electrical input to modulator 12. Once the amplitude of thiselectrical input signal increases above a predetermined level, theoptical output will no longer be able to track the linear input signaland the optical output begins to saturate causing output distortion.This saturation results in the “abrupt” transitions as illustrated inoutput element 24 of FIG. 3(a). The use of pre-emphasis circuit 26, inaccordance with the present invention, functions to boost the extremesof the linear input voltage signal (i.e., pre-emphasis), thus increasingthe linear range of the modulator. The resultant output, as shown inelement 28, thus more closely tracks the shape (and frequency) of theinput. Advantageously, the use of pre-emphasis as a linear channelequalization technique provides an improvement in the optical outputpower, as illustrated in FIG. 3(c), which contains plots of the opticaloutput power as a function of the electrical input power, the dashedcurve associated with the arrangement without pre-emphasis. As shown,there is a gradual, asymptotic curve toward the maximum power for thisarrangement. In contrast, the arrangement of the present inventionincluding pre-emphasis allows for a constant increase in optical outputpower increasing the linear dynamic range, reaching the maximum valuewithout gain compression avoiding signal distortion. Ultimately at somepower level, clipping occurs causing distortion but linearized devicesare not operated at or above the saturation point. In fact, significantlinear range is achieved using pre-emphasis well before reaching thesaturation point. The saturation point can only be increased byincreases in the operating supply voltage.

FIG. 4 illustrates a simplified block diagram for an exemplary MachZehnder interferometer modulator showing the individual accumulation anddepletion arms of the design. Each arm of the interferometer contains aphase modulator device with two terminals. Terminal 1 is made from asilicon (often polysilicon) layer 2 as described in FIG. 1, doped n-typeas region 50 in the accumulation arm and 60 for the depletion arm.Terminal 2 is formed in the SOI layer 3 also shown in FIG. 1, dopedp-type as region 56 in the accumulation arm and 62 for the depletionarm.

FIG. 5 contains a set of reference voltages and drive patternsassociated with the bandwidth-limited channel arrangement of FIG. 2(a)and distortion-limited channel arrangement of FIG. 3(a). The exemplaryoptical data to be transmitted is illustrated along the top trace A inFIG. 5. One arrangement for providing this data pattern as an input tomodulator 12 is for the output from electrical data source 10 to holdone terminal at a pre-defined reference potential (in this case“terminal 2” is held at VDD), shown as trace B. The remaining terminal(“terminal 1”) is then moved about a second reference potential(REF1A/REF1D) to define optical “1” and “0”, as shown in trace C.Referring to FIG. 5, the state of optical “1” is defined when terminal 1of both the accumulation and depletion arms of a modulator of FIG. 4 areheld at this pre-defined reference voltage (i.e., REF1A>REF1D). For anoptical “0”, the depletion arm terminal 1 switches to a higher voltage(REFOD), which may be as high as VDD (but not necessarily equal to VDD),and the accumulation arm terminal 1 switches to a lower voltage (REFOA),which may be as low as VSS (but not necessarily equal to VSS). If thearrangement is driven rail-to-rail (that is, between VDD and VSS), thereis a natural RC time constant that then limits the switching speed ofthe modulator.

As illustrated in FIGS. 2(b) and 3(b), this limitation of the bandwidthlimited and distortion limited channels respectively, can be overcome byadding electrical signal pre-emphasis at each transition between anoptical “0” and an optical “1” (and vice versa), where the pre-emphasiswill accelerate carrier movement and increase switching speed. FIG. 6contains an exemplary set of reference voltages and drive patternsincluding pre-emphasis that can be used to accomplish increasedswitching speed (greater than 1 Gb/s) in accordance with the presentinvention. As with the prior art set of drive patterns shown in FIG. 5,“terminal 2” in the pre-emphasis case can be held at a predeterminedreference potential (e.g., VDD), and the voltage applied to “terminal 1”changed as a function of the data pattern to impose this data pattern onthe propagating optical signal. Also similar to the arrangementassociated with the drive patterns of FIG. 5, the state of optical “1”is defined for the inventive arrangement when terminal 1 of both theaccumulation and depletion arms of a modulator of FIG. 4 are held atessentially the same pre-defined reference voltage (i.e., REF1A≢REF1D).

During the first transition from optical “1” to optical “0” (a fallingedge transition), as shown in FIG. 6, the voltage applied to terminal 1of the depletion arm includes an initial pulse that will over-shoot thevoltage level associated with the steady-state value of optical “0”(REF0D), the initial pulse having a magnitude M_(D10) and time durationt_(D10) sufficient to accelerate the movement of free carriers out ofthe depletion arm device channel. This shaded pulse region isillustrated as “D10” in FIG. 6, where the same pulse D10 will be appliedto the depletion arm for every transition from optical “1” to optical“0”. In a similar fashion, the voltage applied to terminal 1 of theaccumulation arm includes an initial pulse that over-shoots the voltagelevel associated with the steady-state value of optical “0” (REF0A),where the pulse magnitude M_(A10) and time duration t_(A10) are chosento accelerate the accumulation of free carriers in the accumulation armdevice channel. This shaded pulse region is illustrated as “A10” in FIG.6, wherein the same pulse A10 will be applied to the accumulation armfor every (falling edge) transition from optical “1” to optical “0”.

In a similar fashion, pre-emphasis may also be used during thetransition from optical “0” to optical “1”, to again enhance themovement of free carriers and enable higher switching speeds. Referringto FIG. 6, during a transition from optical “0” to optical “1”, anover-shoot pulse, denoted D01 is added to the voltage change needed toswitch the accumulation arm from REF0D to REF1D, where pulse D01 isillustrated as comprising a predetermined magnitude M_(D01) and timeduration t_(D01). The pre-emphasis associated with the accumulation armis illustrated as pulse A01, having a magnitude of M_(A01) and timeduration t_(A01). It is to be noted that each of the various pulsemagnitudes and time durations for the “1” to “0” transition and “0” to“1” transition for both the accumulation and depletion arms may bedifferent, since the movement of free carriers in each state may bedifferent and may require pulses of higher/lower magnitude, and for alonger/shorter time duration. It is an aspect of the present inventionto individually tailor each one of these variables for each case, thusoptimizing the movement of free carriers and enabling high speedoperation for the optical modulator. Moreover, there may be instanceswhere only “rising edge” pre-emphasis is desired, or perhaps only“falling edge” pre-emphasis. All of these cases are considered to fallwithin the scope of the present invention.

FIG. 7 is a top view, in simplified form, of an exemplary Mach-Zehnderelectro-optic modulator 30 formed in accordance with the presentinvention to provide for high speed (greater than 1 Gb/s) operation. Anoptical signal I_(in) is illustrated as traveling along an inputwaveguide 32 and entering an optical splitter 34. As discussed in ourabove-cited co-pending application and with reference to FIG. 1 herein,input waveguide 32 and optical splitter 34 may be formed within the SOIlayer of an SOI wafer (such as SOI layer 3 of FIG. 1). Optical splitter34 may simply divide the optical power present in the signal in half,such that a balanced 50/50 split of signal I_(in) is presented to eacharm of modulator 30. However, any other split may be used, or evendesired, depending on the parameters of the accumulation and depletionarms (parameters such as doping concentration, length of each activearea, materials used to form terminal 1 and terminal 2, etc.). As shownin FIG. 7, a first output from optical splitter 34′ denoted as I_(inA)is thereafter applied as the optical input signal to accumulation arm36, where optical signal I_(inA) propagates along an optical waveguide38 formed along the extent of accumulation arm 36. A second output fromoptical splitter 34, denoted as I_(inD), is thereafter applied as theoptical input signal to depletion arm 40, where optical signal I_(inD)propagates along an optical waveguide 42 formed along the extent ofdepletion arm 40. It is noted that some loss will inevitably occurwithin optical splitter 34, where this loss is denoted by signalI_(loss) in FIG. 7. Subsequent to being modulated within theirrespective active waveguide areas, the modulated optical signalsI_(outA) and I_(outD) will be combined within an optical combiner 44 andthereafter propagate as modulated signal I_(out) along an outputwaveguide 46.

As described in detail in our above-cited co-pending application, theactive waveguiding areas where modulation occurs are formed byoverlapping portions of the SOI layer (such as SOI layer 3 of FIG. 1)and an overlying silicon layer (for example, polysilicon layer 2 of FIG.1), with a relatively thin dielectric layer (layer 5) disposedtherebetween. The dielectric layer is not evident in the top view ofmodulator 30 of FIG. 7, but is visible in the device side view ofFIG. 1. Referring to accumulation arm 36, the “terminal I” material,illustrated as region 50, comprises a portion of silicon (usuallypolysilicon) layer 2 of FIG. 1 that has been doped (for example, “n”doped), where the doping profile may be preferably controlled inaccordance with the present invention to form a lightly doped portionwithin active waveguide area 52, and a more heavily doped portion alongthe terminal 1 electrical contact area 54. The “terminal 2” material,which may comprise SOI layer 3 of FIG. 1, is formed as shown under dopedregion 50 of terminal 1. Terminal 2 SOI region 56 (which is oppositelydoped with respect to the terminal 1 region) is formed to overlap region50 in active waveguide area 52, then extend in the opposite direction toform its contact area 58. As before, the doping density of region 56 ismaintained at a lower level in active waveguide area 52 to minimizeoptical loss (while allowing for a heavier doping density in contactarea 58).

Polysilicon regions 50 and 60, in a preferred embodiment, include inputand output tapered regions to present a graded index change to anoptical signal entering and exiting active waveguide areas 52 and 64,thus minimizing optical reflections at the inputs and outputs of theassociated active waveguide areas. Referring to FIG. 7, polysiliconregion 50 is illustrated as including an input taper 66 and an outputtaper 68, and polysilicon region 60 is illustrated as including an inputtaper 70 and an output taper 72. The input tapers 66 and 70 function togradually increase the effective refractive index of the waveguidelayer, where the gradual tapers will introduce less reflection thansimply disposing a polysilicon layer over the SOI layer and introducingan abrupt change in refractive index to a propagating optical signal. Ina similar fashion, output tapers 68 and 72 will gradually decrease theeffective refractive index. A detailed description of the use of suchtapered polysilicon layers may be found in our co-pending applicationSer. No. 10/818,415, filed Apr. 5, 2004.

In terms of applying the electrical modulating signals, and withreference to FIGS. 6 and 7, SOI regions 56 and 62, being defined as the“terminal 2” inputs for accumulation arm 36 and depletion arm 40,respectively, are coupled, in this particular example, to a referencevoltage (such as VDD in this embodiment). A set of four different inputsignals are shown as coupled (in this embodiment) to the “terminal 1”connections for accumulation arm 36 and depletion arm 40. These signalscorrespond to those discussed above in association with FIG. 6, namely,a “pre-emphasis” pulse for an optical “0” to “1” transition, a referencelevel for optical “1”, a reference level for optical “0”, and apre-emphasis pulse for an optical “1” to “0” transition. In accordancewith the particular data pattern, these various inputs are controlled toapply the proper pre-emphasis signals at each transition, followed bythe proper reference level for the remainder of the duration of theparticular logic level.

FIG. 8 is a preferred embodiment of modulator 30 of FIG. 7, in terms ofthe voltage levels used to define the reference voltages and thepre-emphasis voltages. FIG. 9 is a diagram of the reference voltages anddrive patterns associated with the arrangement of FIG. 8. As shown,“terminal 2” is fixed at a reference voltage potential of VDD (theconventional drain voltage for CMOS applications). The maximum voltagelevel for applying a pre-emphasis pulse to terminal 1 of accumulationarm 36 for an optical “0” to “1” transition is also defined as VDD.Similarly, the VDD voltage level is used to define the maximum level ofa pre-emphasis pulse for a “1” to “0” transition on depletion arm 40. Asalso shown in FIG. 9, the conventional source voltage level VSS is usedto define the maximum value for a pre-emphasis pulse for a “1” to “0”transition on accumulation arm 36 and a “0” to “1” transition ondepletion arm 40. The reference voltage level for a steady-state optical“1” is defined as the midpoint between VDD and VSS, with the referencevoltage for a logic “0” (REFOD) on depletion arm 40 being greater thanthis midpoint value, but less than VDD, and the reference voltage for alogic “0” (REFOA) on accumulation arm 36 being less than this midpointvalue, but greater than VSS.

In silicon, the velocity of light is approximately 0.833×10⁸ m/sec.Since an exemplary modulator of the present invention is about 1 mm inlength (a typical value), the transit time for an optical signal topropagate from the input to the output of the modulator is approximately12 psec. For the relatively high speed applications particularlywell-suited for the invention, 12 psec may become a significant portionof a bit period, thus leading to increases in bit error rate. Therefore,in association with an improved embodiment of the present invention, theelectrical contacts to regions 50 and 60 are disposed in a “fan-out”configuration along the length of the modulator active waveguide area.FIG. 7 illustrates a first plurality of contacts 54 disposed along thelength of region 50 and a second plurality of contacts 82 disposed alongthe length of region 60. If contacts are made to both the “terminal 2”and “terminal 1” regions, a series of contacts may be formed on eachregion as shown in FIG. 7. Each contact or small group of contacts, canbe energized by a separate metal line (not shown) and transistor (alsonot shown) forming a parallel distribution network turning the device onand off along the full length at the same time without propagation delaydifferences between one end of the device and the other. It is to beunderstood, however, that there may be some situations where it isdesired to impart a time delay between the input and output (tointroduce negative chirp into the signal, for example), so in thosecases relatively few, or only one, contact is required.

As mentioned above, there exist fabrication and environmentaldifferences that may affect the performance of an electro-opticmodulator, both in terms of determining the proper reference voltagelevels and in terms of determining the magnitude, polarity and durationof the pre-emphasis pulses. Fabrication variations (such as, forexample, differences in doping density) may be studied at the end of thefabrication process to determine the optimum voltage levels anddurations, with this information stored in a memory element (such as anon-volatile memory look-up table) co-located with the modulator (thememory element being either on-chip or off-chip). More importantly, afeedback arrangement can be used to continuously monitor the opticaloutput from the modulator and adjust one or more of the control signals,as need be, to maintain an optimum output signal. The feedbackarrangement may also provide adaptive, real time updating of the look-uptable constants.

FIG. 10 illustrates, in block diagram form, an exemplary modulatorarrangement of the present invention, including a feedback loop betweenthe modulator output and equalizer/pre-emphasis circuit 18. As shown, aportion of the modulated output signal (preferably, a relatively smallportion) is tapped off from the output and applied as an input to aphotodiode 90, which converts the optical signal into an electricalsignal. In this particular embodiment, the analog electrical output fromphotodiode 90 is then passed through an A/D converter 91 to form adigital feedback signal. The digital feedback signal is then applied asan input to a micro-controller 92. The characteristics of this digitalfeedback signal are then analyzed using digital signal processingtechniques and compared against pre-defined “control” values (which mayhave been stored in a look-up table 94) to assess the performance of themodulator. Indeed, as a function of various environmental changes thatmay take place (temperature variations, supply voltage variations,etc.), it may be necessary to change one or more of the reference valuesapplied to pre-emphasis circuit 18 (including both the magnitude andduration of the pre-emphasis pulse). Therefore, a set of differentreference voltage values associated with various operating conditionsmay also be stored in look-up table 94 and transmitted as adjustmentinputs to pre-emphasis circuit 18. A system interface 96 is included andmay be used to interface with an external control system (not shown) tosend information about circuit adjustments to a centralized recordkeeping facility and/or receive updated information (including, perhaps,changes in algorithms used in the digital control circuit 92) from acentralized control source.

FIG. 11 illustrates another embodiment of a feedback arrangement thatmay be used with an electro-optic modulator formed in accordance withthe present invention. In this case, the tapped-off output signal isagain converted by photodiode 90 into an analog electrical signal. Inthis arrangement, the electrical signal is applied as an input to ananalog feedback circuit 98 that performs one or more analyses on theoutput signal, providing a plurality of outputs that are passed throughan A/D converter 99 and then applied as an input to a digital logicelement 100. The combination of analog feedback circuit 98 and digitallogic element 100 is therefore used to control various ones of theequalizer/pre-emphasis parameters. These parameters include, for examplepre-emphasis magnitude, duration and polarity; reference voltagemagnitudes and accumulation and/or depletion signal magnitudes. Oneparticular implementation, as illustrated in FIG. 11, assigns adifferent low frequency “dither” signal (f₁, f₂, . . . ) to eachparameter to be controlled. Analog feedback circuit 98 is then used toseparate out each of these selected control frequencies, creating a setof control “signature” signals prior to applying them as an input todigital logic element 100. Analog feedback circuit 98 also conditionseach of the control channels with an appropriate loop time constant toeliminate instabilities. After digitizing through A/D converter 99, thecontrol signals are provided as an input to a system of digital logicgates within digital logic element 100, the gates configured to maximize(or minimize) a particular control signal. The output of the logic gatesthen applies the proper modification to the equalizer parameters, usinga prescribed algorithm (that may either be fixed or adaptive). Theoutput from digital logic element 100 may also be applied as an inputthrough a system interface 102 to an external control system (not shown)for error reporting and/or for the installation of new, updatedalgorithms. The values presented to digital logic element 100 are thenused to determine the changes to voltage levels and/or pulse durationsto be used by pre-emphasis circuit 18.

It is to be understood that the pre-emphasis technique of the presentinvention is applicable to any type of silicon-based electro-opticmodulator. In terms of a Mach-Zehnder interferometer, the technique canbe used with symmetric interferometers (i.e., 50:50 split of inputoptical signal along each arm) as well as for asymmetric interferometers(unequal split). Although the particular arrangements described aboveheld one terminal (in this case, “terminal 2”) at a constant referencevalue while changing the reference value of the remaining terminal, itis also possible to apply different voltage levels and offsets to eachterminal to generate the same pre-emphasis pulses, as well as theoptical “1” and optical “0” output values. Indeed, the arrangement ofthe present invention is equally applicable for use with linearmodulators as with nonlinear, digital devices. Regarding the use of thepre-emphasis technique with other types of modulators, FIG. 12illustrates an exemplary absorption-based modulator that may use thepre-emphasis technique of the present invention to increase theswitching speed of the modulator.

An electro-absorption modulator can be formed using a MOSCAP devicedriven such that the free carrier absorption is maximized. Absorptioncan be controlled by a modulating electrical voltage such that, underthe correct conditions, a “0” (or low) modulating signal causes theoptical signal from an optical source to be partially absorbed(accumulation state) by the modulator, and a “1” (or high) modulatingsignal causes the modulator to allow the signal to pass throughsubstantially unabsorbed (depletion state). A DC optical input signalwill therefore either be substantially absorbed or not absorbed, as afunction of the electrical data signal input, generating a modulatedoptical output signal. FIG. 12(a) illustrates an exemplaryelectro-absorption modulator 110 of the present invention in theaccumulation state. Electro-absorption modulator 110 includes an inputwaveguide 120, similar to the modulators described above, where inputwaveguide 120 comprises a selected portion of the relatively thin SOIlayer of an SOI structure. The DC optical input signal, I_(in), isapplied as an input to waveguide 120. An active waveguide area 122 isformed, in this case, by the overlap of a section of doped silicon(usually, polysilicon) layer 124 (defined as “terminal I” material) witha section of doped SOI material 126 (defined as “terminal 2” material).In this embodiment, a thin dielectric layer is disposed between theselayers (not visible in the top view illustration of FIG. 12).

To obtain an optical “0” output in this particular configuration,terminal 2 is held at a pre-determined reference value (e.g., VDD), withthe terminal 1 electrical contact set to a value associated with theaccumulation state that will absorb a sufficient quantity of the opticalsignal. The optical output from FIG. 12(a) is thus illustrated asI_(out0). FIG. 12(b) illustrates electro-absorption modulator 110 in thedepletion state, with the application of a voltage associated withallowing the optical signal to propagate along active waveguide area 122essentially unchanged, the output thus representative of an optical “1”and denoted as I_(out1). FIG. 13 is a graph of the reference voltagesand drive patterns associated with the arrangement of FIG. 12. Inaccordance with the present invention, during a transition between anoptical “1” and optical “0” (i.e., a falling edge transition), therelatively high voltage associated with maintaining the optical “1”state is dropped to the VSS rail, in the form of a pulse lasting for apredetermined time duration t_(A10) (pulse A10). At the end of thispulse, the reference voltage (REF0) applied to the “terminal 1” materialof section 124 is then maintained at a relatively low reference voltageassociated with the optical “0” value, this voltage being slightlygreater than VSS. In a similar fashion, during a transition between anoptical “0” and an optical “1” (leading edge transition), the voltagewill be increased to the VDD rail, for a pulse of duration t_(D01)(pulse D01) before returning to the voltage level (REF1) associated withmaintaining the optical “1” value. Thus, as with the case of theelectro-optic interferometer, an electro-absorption modulator may alsoprovide increased switching speeds, in accordance with the presentinvention, by accelerating the movement of free carriers during theinitial transition between logic levels.

Various other modulator improvement techniques discussed above inassociation with the Mach-Zehnder interferometer are equally applicablefor use with the electro-absorption modulator. For example, the dopingprofile within regions 124 and 126 can be controlled to provide forrelatively light doping in active waveguide area 122 (preferred tominimize optical loss) and relatively heavy doping in the contact areas(preferred to maximize switching speed). Further, optical reflections atthe input and output of active waveguide area 122 can be minimized byincluding tapers in the topography of polysilicon area 124, the tapersintroducing a gradual change in the effective index seen by an opticalsignal propagating through the active waveguide area. Moreover, transittime skew problems may be addressed by utilizing a plurality of contactregions formed along the length of the terminal 1 contact (region 124)and terminal 2 (region 126).

FIGS. 14 and 15 contain plots illustrating the improvement in switchingspeed of an electro-optic modulator using equalization/pre-emphasis inaccordance with the present invention. The values illustrated in FIG. 14are simulated values associated with the accumulation case and thevalues illustrated in FIG. 15 are simulated values associated with thedepletion case. Indeed, these values may be compared to the ideal caseshown in FIG. 6. For the nonlinear accumulation case shown in FIG. 14,the voltage applied to “terminal 2” (the SOI layer of a modulatorstructure) is constant, in this example at a value of 1.7V as shown bycurve A. For an arrangement without pre-emphasis, the modulating voltageapplied to the polysilicon “terminal 1” is illustrated as curve B,switching between an optical “1” value of 0.65V and an optical “1” valueof 0.35V. Curve C illustrates the same modulating voltage, in this caseincluding pre-emphasis in accordance with the teachings of the presentinvention. In this case, the pre-emphasis pulse is selected to have amagnitude such that the pulse reaches VSS before returning to thesteady-state optical “1” value of 0.35V. As shown, the addition of thepre-emphasis results in an increased terminal 1 charging current withshorter decay time resulting from the larger dv/dt (illustrated as curveD). After removal of the pre-emphasis pulse, the terminal 1 chargingcurrent returns to zero, which indicates reaching the desired opticalstate within the bit interval. For this example, the “1” to “0” and “0”to “1” pre-emphasis voltage magnitude and duration are equal. This isnot necessarily the case.

FIG. 15 contains similar results for the nonlinear depletion case, whereagain the voltage applied to the SOI layer (terminal “2” in the previousfigures) is held at a value of 1.7V (that is, essentially the same valueas used for the accumulation arm, as discussed above). The switchingvoltage applied to terminal “1”, labeled as curve B, is shown to risefrom a value of 0.7V to 1.3V. It is to be noted that a voltageapproximately twice in magnitude to that associated with theaccumulation case is required to generate the same free carrier changeto achieve approximately π/2 radian phase shift in each arm of themodulator shown in FIG. 4. Curve C illustrates a modified voltage to beapplied to terminal “1”, including a pre-emphasis pulse as proposed inaccordance with the present invention. The pulse as illustrated in FIG.15 has a magnitude that brings the optical “1” voltage applied toterminal 1 for the depletion case essentially equal to the optical “1”voltage applied to terminal 1 for the accumulation case shown in FIG.14. This is not necessarily the case. For this example, the “1” to “0”and “0” to “1” pre-emphasis voltage magnitude and duration are notequal, representing a more general case. The resulting current plot, asshown in curve D, like the current plot of FIG. 14, illustrates animprovement in terms of steeper rise and fall times, with very littlenoticeable overshoot, all indications of the speed improvement that canbe achieved by using pre-emphasis in accordance with the presentinvention.

The significant improvement in switching speed as a result of usingpre-emphasis in accordance with the present invention is also evident bythe graph of FIG. 16, which illustrates the change in carrierconcentration as a function of time. Without using pre-emphasis, it isclear that for both the rising edge and falling edge there is asignificant time delay, with the neither full optical “1” or “optical“0” free carrier concentrations being obtained. In contrast, for anarrangement using pre-emphasis in accordance with the present invention,the delays on both the rising edge and falling edge are significantlyreduced, with both the optical “1” and optical “0” levels being reached,and maintained, for a significant portion of the bit period.

As other embodiments of the invention will occur to those skilled in theart, the scope of the present invention is to be defined by the terms ofthe following claims and recognized equivalents. For example, the p-typedoping of the SOI layer and the n-type doping of the overlying siliconlayer may be interchanged, with the appropriate reversals in thepolarity of the applied voltages. Additionally, there may be cases wherepre-emphasis is only required on a rising edge of the data pattern, oronly on the falling edge (for the nonlinear case). Moreover, asmentioned above, the technique of the present invention is equallyapplicable to a system utilizing a linear (e.g., AM) input data signal.In summary, therefore, the scope of the present invention is consideredto be limited only by the scope of the claims appended hereto.

1. An arrangement for generating a high speed optical output signalmodulated by an input data pattern, the arrangement comprising anelectro-optic modulator responsive to an optical input signal and amodulating electrical input signal for generating the modulated opticaloutput signal, the electro-optic modulator comprising a freecarrier-based modulator utilizing changes in carrier density to generatethe modulated optical output signal; an equalizer/pre-emphasis moduledisposed at the electrical input to the electro-optic modulator, theequalizer/pre-emphasis module for inserting a first pre-emphasis pulseof a predetermined magnitude and a predetermined duration into themodulating electrical input signal at each transition between a firstdata value and a second data value of the input data pattern andinserting a second pre-emphasis pulse of a predetermined magnitude and apredetermined duration at each transition between the second data valueand the first data value the second pre-emphasis pulse having a polarityopposite to the first pre-emphasis pulse, the inserted first and secondpre-emphasis pulses for extending the bandwidth of the electro-opticmodulator without increasing the optical loss thereof; and a controlmodule responsive to a portion of the modulated optical output signalfor measuring the modulated optical output signal and determiningoptimum values for at least one operating parameter from the set of: thefirst pre-emphasis pulse duration, the second pre-emphasis pulseduration, the first pre-emphasis pulse magnitude and the secondpre-emphasis pulse magnitude, the control module including a feedbackelement to continuously measure a portion of the modulated opticaloutput signal and update the magnitude and duration values of the firstand second pre-emphasis pulses in association with changing operatingconditions, the control module further comprising a look-up tableincluding listings of pre-emphasis pulse magnitude and duration valuesassociated with changing operating conditions.
 2. The arrangement asdefined in claim 1 wherein the output from the electro-optic modulatoris a linear, analog signal.
 3. The arrangement as defined in claim 1wherein the output from the electro-optic modulator is a nonlinear,digital signal. 4.-18. (canceled)
 19. The arrangement as defined inclaim 14 wherein the control module supplies the determined optimumvalues to the equalizer/pre-emphasis arrangement to set the operatingcharacteristics of the inserted first and second pre-emphasis pulses atthe completion of the fabrication process. 20.-21. (canceled)
 22. Thearrangement as defined in claim 1 wherein the control module is adaptivewith respect to real-time updating of the pre-emphasis pulse magnitudeand duration values stored in the look-up table.
 23. The arrangement asdefined in claim 1 wherein the control module comprises an interface toaccept updates for the look-up table values from an external source.24.-35. (canceled)
 36. The arrangement as defined in claim 1 wherein theelectro-optic modulator is an interferometer including an opticalsplitter disposed at the modulator input to divide the optical inputsignal into a first arm and a second arm; a first modulation elementdisposed along the first arm, the first modulation element having afirst region of a first conductivity type and a second region of asecond conductivity type; a second modulation element disposed along thesecond arm, the second modulation element having a first region of thefirst conductivity type and a second region of the second conductivitytype; and an optical combiner disposed at the modulator output tocombine the modulated optical output signals from the first and secondarms, wherein the modulating electrical input signal is applied to atleast one of the first and second modulation elements to generate themodulated optical output signal. 37.-46. (canceled)
 47. The arrangementas defined in claim 36 wherein the modulating electrical input signal,in association with the input data pattern, comprises a first referencevoltage (REFOA) representative of a logic “0” value for the firstmodulation element, a second reference voltage (REFOD) representative ofa logic “0” for the second modulation element, a third reference voltage(REF1A) representative of a logic “1” value for the first modulationelement, and a fourth reference voltage (REF1D) representative of alogic “1” value for the second modulation element, with a non-modulatingpotential voltage (REF) applied to one region of each modulationelement. 48.-57. (canceled)
 58. The arrangement as defined in claim 47wherein the the control module, responsive to the modulated opticaloutput signal, functions to adjust at least one modulator parameterselected from the group consisting of: the first, second, third andfourth reference voltages, the non-modulating potential voltage, thepre-emphasis pulse magnitude and the pre-emphasis pulse duration tooptimize modulator performance as a function of time.
 59. Thearrangement as defined in claim 58 wherein a separate low frequencycontrol signal is applied to each selected modulator parameter and thecontrol module further comprises an analog feedback element forseparating out by filtering each low frequency control signal componentpresent in the modulated optical output signal, the control module foranalyzing the recovered low frequency signals to determine adjustmentsto the specific modulator parameters associated with certain lowfrequency signals.
 60. The arrangement as defined in claim 58 whereinthe control module further comprises a photodetector for capturing aportion of the modulated optical output signal and converting thecaptured portion into an analog electrical feedback signal; an A/Dconverter for converting the analog electrical feedback signal into aplurality of digital feedback signals; and a digital logic unit coupledto the output of the A/D converter, wherein the plurality of digitalfeedback signals are analyzed by the digital logic unit using digitalsignal processing techniques.
 61. The arrangement as defined in claim 60wherein the digital signal processing techniques include rapidconvergence algorithms for one or more loop equations for each selectedparameter.
 62. The arrangement as defined in claim 58 wherein thecontrol module further comprises an interface for communicating with anexternal source to update the processes of the digital logic unit. 63.The arrangement as defined in claim 62 wherein the control moduletransmits and receives updated information through the interface to areporting device at an external source.
 64. The arrangement as definedin claim 58 wherein the control module further comprises a photodetectorfor capturing a portion of the modulated optical output signal andconverting the captured portion into an analog electrical feedbacksignal; an A/D converter coupled to the photodetector for converting theanalog electrical feedback signal into a digital electrical feedbacksignal; a control element responsive to the digital electrical feedbacksignal to determine changes in modulated optical output signal quality;and a look-up table coupled to the control element, the look-up tableincluding listings of a plurality of different values for each modulatorparameter for a plurality of different operating conditions, wherein thecontrol element uses determined changes in the modulated optical outputsignal to find appropriate modulator parameter values from the look-uptable.
 65. The arrangement as defined in claim 1 wherein theelectro-optic modulator is an absorption modulator comprising a firstsemiconductor element doped with a first conductivity type; and a secondsemiconductor element doped with a second conductivity type, with arelatively thin dielectric layer disposed between, wherein theapplication of the modulating electrical input signal causes the opticalinput signal to be partially absorbed by the first and second elementsfor the optical “0” state and to be essentially unabsorbed for theoptical “1” state, generating the modulated optical output signal, thepre-emphasis pulse thus accelerating the change between the absorbingstate and the unabsorbing state. 66.-68. (canceled)
 69. The arrangementas defined in claim 65 wherein the arrangement further comprises acontrol module responsive to a portion of the modulated optical outputsignal for measuring the modulated optical output signal and determiningoptimum values for at least one operating parameter selected from theset of: the first pre-emphasis pulse duration, the second pre-emphasispulse duration, the first pre-emphasis pulse magnitude and the secondpre-emphasis pulse magnitude.
 70. The arrangement as defined in claim 69wherein the control module supplies the determined optimum values to theequalizer/pre-emphasis arrangement to set the operating characteristicsof the inserted first and second pre-emphasis pulses at the completionof the absorption modulator fabrication process.
 71. The arrangement asdefined in claim 69 wherein the control module includes a feedbackelement to continuously measure a portion of the modulated opticaloutput signal and update the magnitude and duration values of the firstand second pre-emphasis pulses in associated with changing operatingconditions.
 72. The arrangement as defined in claim 69 wherein thecontrol module further comprises a look-up table including listings ofpre-emphasis pulse magnitude and duration values associated withchanging operating conditions.
 73. A free-carrier based electro-opticinterferometer comprising a first arm including a first opticalwaveguide; a second arm including a second optical waveguide; an opticalsplitter for dividing an optical input signal into a first input signalto be coupled into the first arm and a second input signal to be coupledinto the second arm; a first free carrier-based modulation elementdisposed along the first arm, the first modulation element having afirst region of a first conductivity type and a second region of asecond conductivity type and utilizing changes in carrier density togenerate a first modulated optical output signal, wherein theapplication of the modulating electrical input signal to the first freecarrier-based modulation element generates free carrier movement so asto modulate the free carrier density in the first and second regions andintroduce a modulation in the refractive index of the waveguide, therebycreating the first modulated optical output signal; and a second freecarrier-based modulation element disposed along the second arm having afirst region of the first conductivity type and a second region of thesecond conductivity type and utilizing changes in carrier density togenerate a second modulated optical output signal, wherein theapplication of the modulating electrical input signal to the second freecarrier-based modulation element generates free carrier movement so asto modulate the free carrier density in the first and second regions inthe second modulation element and introduce a modulation in therefractive index of the waveguide, thereby creating the second modulatedoptical output signal; an optical combiner disposed at the output of thefirst and second arms to combine the first and second modulated opticaloutput signals from the first and second arms, wherein the electricalmodulating signal is applied to at least one of the first and secondmodulation elements to form a modulated optical output signal; anelectrical signal source for generating a first reference voltage(REF0A) representative of a logic “0” value for the first modulationelement, a second reference voltage (REF0D) representative of a logic“0” value for the second modulation element, a third reference voltage(REF1A) representative of a logic “1” value for the first modulationelement, a fourth reference voltage (REF1D) representative of a logic“1” value for the second modulation element, and a non-modulatingpotential voltage (REF) to be applied to one region of each modulationelement; and a feedback module, responsive to the modulated opticaloutput signal, to adjust at least one modulator parameter selected fromthe group consisting of: the first, second, third and fourth referencelevels, and the non-modulating potential voltage to optimizeinterferometer performance as a function of time. 74.-86. (canceled) 87.The arrangement as defined in claim 73 wherein a separate low frequencycontrol signal is applied to each selected modulator parameter and thefeedback module further comprises a filter for separating out each lowfrequency control signal component present in the modulated opticaloutput signal, the feedback module for analyzing the recovered lowfrequency signals to determine adjustments to the selectedinterferometer parameters.
 88. The arrangement as defined in claim 73wherein the feedback module further comprises an A/D converter forgenerating a plurality of digital feedback signals, wherein theplurality of digital feedback signals are analyzed by the feedbackmodule using digital signal processing techniques.
 89. The arrangementas defined in claim 1 wherein the output impedance of theequalizer/pre-emphasis module approaches an ideal voltage so as tominimally impact the switching speed of the electro-optic modulator.